This invention relates to an abnormality detection apparatus for vehicle engines especially for detecting abnormality in a vehicle speed sensor system used for controlling the vehicle engine.
In the vehicle engine, a signal from a vehicle speed sensor (vehicle speed signal) has conventionally been used as one of important parameters for controlling the engine. For example, the vehicle speed signal is used for deciding an idle state in controlling the number of idling rotations.
In the event that a signal line for transmitting the vehicle speed signal to a control computer is broken or a connector for the signal line is accidentally disconnected with the result that the transmission of the speed signal to the control computer is prevented, troubles will be raised in various modes of engine control. For example, in the feedback control of the idle rotation number, the idle state becomes indistinguishable from a deceleration state and even the deceleration state is erroneously decided to be the idle state, with the result that the amount of air is decreased to decrease the rotation number and consequently engine stall tends to occur. Further, a speed limiter, provided for performing fuel cut when the vehicle speed exceeds a predetermined value, will become unable to function. In view of the above, an abnormality detecting apparatus for detecting breaking of the speed signal line has conventionally been proposed, as disclosed in, for example, Japanese Laid Open Publication No. 59-165841. According to the detector described in this publication, when an engine rotation number (N) in excess of a predetermined value, an engine load (Q/N) in excess of a predetermined value and a vehicle speed short of a predetermined value are all detected, the speed signal line is decided to be broken.
The detector described in the aforementioned publication is however disadvantageous in that when the discrimination levels for the engine rotation number (N) and the engine load (Q/N) are high (large), the detector is not used frequently to decrease its operating ratio and that conversely, when both the discrimination levels are low (small), the detector tends to face a failure to distinguish the breaking of the speed signal line from the stopping of the vehicle and there is a possibility that errors in detection occur. Because of the above problems, it is difficult for the prior art detector to obtain compatibility of detection accuracy and operating ratio. In addition, when no-load high speed rotation (racing) is carried out in cold weathers by the driver immediately after the engine has started, the requirements prescribed by the aforementioned publication for giving a decision on the breaking of the speed signal line may sometimes be satisfied, thus also inviting the possibility of the occurrence of an erroneous detection.